Shock wave lithotripters utilize high-energy focused shock waves to disintegrate concretions located in the upper urinary track and kidney of a patient. Recent studies have demonstrated that a lithotripter with a broad beam size (defined by the −6 dB of the peak pressure distribution in the focal plane of the lithotripter) can generate better stone comminution than its counterpart with a narrow beam size under the same effective acoustic pulse energy. This observation can be attributed to several factors, including lateral spreading of residual stone fragments, stone movement due to respiratory motion, and practical difficulties in accurate alignment of the stone to lithotripter focus during clinical treatment. Hence, broadening the traverse beam size of the lithotripter in its focal plane can benefit stone comminution.
However, enlargement of the traverse beam size in the focal plane is generally limited by the simultaneous increase in the longitudinal beam size of the lithotripter along the axis of the incident shock wave. This later parameter determines the pressure amplitude at the patient's flank, and therefore correlates with discomfort and skin lesion produced at the shock wave entrance/exit sites during clinical shock wave lithotripsy (SWL). Independent of the design, all modern clinical shock wave lithotripters produce an axisymmetric acoustic field around the central axis of the shock source. As a result, enlargement of the transverse beam size is severely limited in current clinical shock wave lithotripters.
To date, no practical methods have been developed to solve this problem. Interestingly, the pressure distribution in the focal plane of the original Dornier HM-3TM lithotripter (Friedrichshafen; W. Germany) is also non-axisymmetric with a broader beam size in the head-foot direction (˜12 mm) and a narrower one in the transverse direction (˜9 mm). This non-axisymmetric pressure distribution is presumably caused by the truncation of the ellipsoidal reflector at the lateral sides to accommodate the bi-planar fluoroscopy for stone localization, which may contribute to the effectiveness of the HM-3. However, the acoustic field in an HM-3 lithotripter cannot be controlled to steer in a designated orientation. Moreover, the eccentricity of the pressure field in an HM-3 lithotripter cannot be adjusted except for some random variations caused by the non-repeatable spark discharge at the tip of the HM-3 electrode.
Therefore, it would be beneficial to provide a lithotripter that is able to enlarge the effective transverse beam size without increasing the longitudinal beam size of the shock wave lithotripter. It would also be beneficial to provide a lithotripter that is steerable.